Hi.
There are other standard names that call for a separate attribute or
variable that provides context. The attributes (at the moment) are all
standard CF attributes (cell_methods, flag_meanings, comment, etc). I'd
love to get feedback from the community about whether or not a
directionality attribute would need to described as a "standard" CF
attribute.
I'll be glad to rework the definitions to make them directionality-agnostic
when I get back next week.
Grace and peace,
Jim
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On Sat, Aug 4, 2018 at 3:45 AM Lowry, Roy K. <rkl at bodc.ac.uk> wrote:
> Dear Nan,
>
>
> So are we returning to the wording in Alison's original definitions (e.g.
> yaw normally clockwise facing front) before you with my support asked for
> the ambiguity be removed? Or do you want to go even further with no mention
> of sign convention at all?
>
>
> I would also question whether a Standard Name definition is the place to
> specify the mechanism to be used for the description of a sign convention
> as it has wider implications than the parameters currently under
> discussion. Would it not be more appropriate for this to be considered an
> enhancement to CF and written into the Conventions document? If so, it
> should it not be the subject of a GitHub ticket?
>
>
> Cheers, Roy.
>
>
> I have now retired but will continue to be active through an Emeritus
> Fellowship using this e-mail address.
>
>
> ------------------------------
> *From:* CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Nan
> Galbraith <ngalbraith at whoi.edu>
> *Sent:* 04 August 2018 02:18
> *To:* cf-metadata at cgd.ucar.edu
> *Subject:* Re: [CF-metadata] Platform Heave
>
> Thanks, Jim.
>
> > change the definitions to avoid declaring which direction is
> > positive, make the direction attribute optional, and say that users
> > should be careful about assuming the directionality for variables
> > lacking the attribute.
>
> This is the approach I'd prefer as well.
>
> - Nan
>
>
> Quoting Jim Biard <jbiard at cicsnc.org>:
>
> > Nan,
> >
> > I didn't go to the lengths of making new regularized definitions
> > before I wrote that list. I was thinking in terms of making the
> > clockwise/anticlockwise call based on the right hand rule and the
> > unit vector for each axis. For roll, for example, if the X unit
> > vector faces forward, the "right side down" roll is actually
> > anticlockwise - that is, it is in the direction that your right hand
> > fingers curl if you grab the unit vector in your hand with your
> > thumb pointing in the same direction as the unit vector. That
> > definition is independent of observer location and look direction.
> > My definitions for all the direction values are following that same
> > convention.
> >
> > Accurate knowledge of the sign of roll, pitch, and yaw is critical
> > in the satellite and aircraft world. The look angles for remote
> > sensors are affected by these values. I get it that not all systems
> > care about the signed values, so that reason and for backward
> > compatibility I suggested that we could change the definitions to
> > avoid declaring which direction is positive, make the direction
> > attribute optional, and say that users should be careful about
> > assuming the directionality for variables lacking the attribute.
> >
> > Grace and peace,
> >
> > Jim
> >>
> >> On 8/3/18 2:03 PM, Nan Galbraith wrote:
> >>> Hi Roy -
> >>>
> >>> Yes, I've been looking at that page quite a bit lately, and I
> >>> think it backs up
> >>> what I'm saying.
> >>>
> >>> If you are standing on that fuselage (may we never), facing
> >>> forward, the red roll
> >>> arrow is showing a clockwise motion, with right side moving
> >>> downward. If you
> >>> were facing aft, the arrow would be anticlockwise, but the right side
> would
> >>> be rising.
> >>>
> >>> So, 'roll: "clockwise" for positive right side up and
> >>> "anticlockwise" for positive right
> >>> side down' - is backwards in either case. I'm not disputing
> >>> anything except
> >>> the term 'clockwise' in this phrase.
> >>>
> >>> Thanks - Nan
> >>>
> >>> On 8/3/18 1:43 PM, Lowry, Roy K. wrote:
> >>>>
> >>>> Hi Nan,
> >>>>
> >>>>
> >>>> Whilst I appreciate the limitations of Wikipedia as an
> >>>> authoritative source have a look at
> >>>> https://en.wikipedia.org/wiki/Aircraft_principal_axes
> <https://en.wikipedia.org/wiki/Aircraft_principal_axes>
> Aircraft principal axes - Wikipedia
> <https://en.wikipedia.org/wiki/Aircraft_principal_axes>
> en.wikipedia.org
> Normal axis, or yaw axis ? an axis drawn from top to bottom, and
> perpendicular to the other two axes.Parallel to the fuselage station.;
> Transverse axis, lateral axis, or pitch axis ? an axis running from the
> pilot's left to right in piloted aircraft, and parallel to the wings of a
> winged aircraft.
>
>
> >>>>
> >>>>
> >>>> Cheers, Roy.
> >>>>
> >>>>
> >>>>
> ------------------------------------------------------------------------
> >>>> *From:* Nan Galbraith <ngalbraith at whoi.edu>
> >>>> *Sent:* 03 August 2018 18:23
> >>>> *To:* Jim Biard; Lowry, Roy K.; kkehoe at ou.edu
> >>>> *Subject:* Re: [CF-metadata] Platform Heave (pitch, roll)
> >>>> Hi Jim, Roy and Ken -
> >>>>
> >>>> I'm skipping the list because this is a minor point and ... and I may
> be
> >>>> missing
> >>>> something obvious.
> >>>>
> >>>> It's hard not to think of these terms as they apply to ships. In that
> >>>> environment,
> >>>> we'd use the convention of the observer facing forward; therefore roll
> >>>> would be
> >>>> clockwise if the right side were going down, not up. I'm supposing
> that
> >>>> would
> >>>> also apply to aircraft.
> >>>>
> >>>> Cheers - Nan
> >>>>
> >>>>
> >>>>
> >>>>> If we declare that X is positive forward, that Y is positive left,
> >>>>> that Z is positive up, and that we use the right-hand rule for angle
> >>>>> directions, the direction attribute values could be:
> >>>>>
> >>>>> * roll: "clockwise" for positive right side up and "anticlockwise"
> >>>>> for positive right side down.
> >>>>> * pitch: "clockwise" for positive nose up and "anticlockwise" for
> >>>>> positive nose down.
> >>>>> * yaw: "clockwise" for positive nose right and "anticlockwise" for
> >>>>> positive nose left.
> >>>>> * surge: "positive" for positive forward and "negative" for
> >>>>> positive backward.
> >>>>> * sway: "positive" for positive left and "negative" for positive
> right.
> >>>>> * heave: "positive" for positive up and "negative" for positive
> down.
> >>>>>
> >>>>
> >>>>
> >>>>>
> >>>>>>>
> >>>>>>>
> ------------------------------------------------------------------------
> >>>>>>> >>>
> >>>>>>> *From:* CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf
> of
> >>>>>>> Jim Biard <jbiard at cicsnc.org>
> >>>>>>> *Sent:* 03 August 2018 15:41
> >>>>>>> *To:* cf-metadata at cgd.ucar.edu
> >>>>>>> *Subject:* Re: [CF-metadata] Platform Heave
> >>>>>>>
> >>>>>>> I freely admit that I picked direction on sway arbitrarily. In my
> >>>>>>> experience, part of the variation that arises in the definitions of
> >>>>>>> the different motions arises from different thoughts about their
> >>>>>>> use, particularly whether someone is thinking the values are used
> to
> >>>>>>> transform into the platform body frame vs transform from it. Or
> >>>>>>> maybe they just aren't worrying about consistency. Like as not,
> >>>>>>> choices have often been made in attempts to make the values have
> the
> >>>>>>> signed-ness that felt right to people, and we can't keep to
> >>>>>>> conventions like the right hand rule and make it all work
> >>>>>>> consistently. We want a positive pitch to be nose up. We want a
> >>>>>>> positive yaw to be nose right. We want positive heave to be up. My
> >>>>>>> natural tendency is to think of "roll right" as positive and "sway
> >>>>>>> right" as positive, but that isn't what other people think of.
> >>>>>>>
> >>>>>>> As I read what I wrote, I realize I didn't use a consistent
> approach
> >>>>>>> to position and look direction when assigning clockwise and
> >>>>>>> anticlockwise to roll, pitch, and yaw. I need to regularize that.
> >>>>>>>
> >>>>>>> Reading the Conventions about vertical coordinates, it says they
> >>>>>>> must all have a "positive" attribute with a value of "up" or
> "down".
> >>>>>>> I don't see a problem with having the definitions back off of
> >>>>>>> declaring a specific directionality and add an attribute declaring
> >>>>>>> directionality. We could call the attribute "direction" so as not
> to
> >>>>>>> step on the "positive" attribute, and say that if the attribute is
> >>>>>>> not present that the user should not assume which direction is
> correct.
> >>>>>>>
> >>>>>>> If we declare that X is positive forward, that Y is positive left,
> >>>>>>> that Z is positive up, and that we use the right-hand rule for
> angle
> >>>>>>> directions, the direction attribute values could be:
> >>>>>>>
> >>>>>>> * roll: "clockwise" for positive right side up and
> "anticlockwise"
> >>>>>>> for positive right side down.
> >>>>>>> * pitch: "clockwise" for positive nose up and "anticlockwise" for
> >>>>>>> positive nose down.
> >>>>>>> * yaw: "clockwise" for positive nose right and "anticlockwise"
> for
> >>>>>>> positive nose left.
> >>>>>>> * surge: "positive" for positive forward and "negative" for
> >>>>>>> positive backward.
> >>>>>>> * sway: "positive" for positive left and "negative" for positive
> >>>>>>> right.
> >>>>>>> * heave: "positive" for positive up and "negative" for positive
> down.
> >>>>>>>
> >>>>>>> Thoughts?
> >>>>>>>
> >>>>>>> BTW, I'll be out until August 13.
> >>>>>>>
> >>>>>>> Grace and peace,
> >>>>>>>
> >>>>>>> Jim
> >>>>>>>
> >>>
> >>>
> >>
> >> --
> >> CICS-NC <http://www.cicsnc.org/> Visit us on
> >> Facebook <http://www.facebook.com/cicsnc> *Jim Biard*
> >> *Research Scholar*
> >> Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/
> >
> >> North Carolina State University <http://ncsu.edu/>
> >> NOAA National Centers for Environmental Information <
> http://ncdc.noaa.gov/>
> >> /formerly NOAA?s National Climatic Data Center/
> >> 151 Patton Ave, Asheville, NC 28801
> >> e: jbiard at cicsnc.org <mailto:jbiard at cicsnc.org <jbiard at cicsnc.org>>
> >> o: +1 828 271 4900
> >>
> >> /Connect with us on Facebook for climate
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> >>
> >>
> >
> > --
> > CICS-NC <http://www.cicsnc.org/> Visit us on
> > Facebook <http://www.facebook.com/cicsnc> *Jim Biard*
> > *Research Scholar*
> > Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
> > North Carolina State University <http://ncsu.edu/>
> > NOAA National Centers for Environmental Information <
> http://ncdc.noaa.gov/>
> > /formerly NOAA?s National Climatic Data Center/
> > 151 Patton Ave, Asheville, NC 28801
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>
>
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